The creation many years ago of brushless alternating current generators in which field excitation is supplied from an alternating current exciter and a rotating rectifier assembly have always faced the inherent problem of detecting a shorted diode in the rotating rectifier assembly. The presence of a shorted diode in the rectifier assembly establishes an intermittent short-circuit across the exciter winding and if the exciter field excitation is maintained, heavy short-circuit currents will flow in the exciter armature winding. These heavy short-circuit currents can result in extensive damage or destruction of the exciter and possible damage to the main generator and other system components.
Amongst the effective solutions to these shorted diode problems is that put forward in the Calfee et al U.S. Pat. No. 3,210,603. The Calfee et al patent shows a rotating field generator in which the exciter field current is monitored and the field circuit interrupted upon failure of a rotating diode. A current transformer couples a signal representing the current to a diode bridge rectifier and RC filter. Upon failure of a diode, the AC ripple in the field circuit increases in amplitude and decreases in frequency. The rectified voltage from the diode bridge rectifier and RC filter increases in amplitude and actuates a relay to open the field circuit. The aforementioned is additionally effective to isolate the direct current component of the field current from the alternating current component. It is recognized that the design of the current transformer is critical since the transformer is biased with direct current. The direct current bias being equal to the average exciter field amperes. This direct bias requires a heavy and costly transformer. The invention to be described hereinafter requires no transformer and is free from any of the problems of isolation attendant with the detection of a current component.
Another solution to the shorted diode problem is that set forth in the Hyvarinen et al U.S. Pat. No. 3,534,228, which shows a sensing circuit including a series resistor in the exciter field circuit with a voltage transformer connected across the resistor through a DC blocking capacitor. The voltage output of the transformer is connected through an RC filter to a diode bridge rectifier and to a field current regulator. The Hyvarinen et al patent requires an AC conductive coupling circuit to the exciter field, to produce an AC error voltage proportional to the oscillation or ripple in the field current. Shorted diode protection is afforded by means for limiting the exciter field current when an AC error voltage exceeds a reference. The invention to be described hereinafter provides shorted diode protection without the need for the inclusion of heavy current components in the field circuit lines thereby reducing the possibility, overall, of component failure as a factor in the reliability of the shorted diode detection.
Yet another solution to the shorted diode protection is the differential protection system shown and described in the South, et al U.S. Pat. No. 3,705,331. South, et al is directed to a differential protection system for an A.C. generator in which there is provided exciter fault detection based on a comparison of signals from the exciter and from the generator armature. A signal representing the voltage across two phases of the armature is connected in series with a signal representing the current through the third phase. The vectorial sum of these two signals is rectified and filtered, providing a measure of real power delivered by the generator. The generator power signal is summed with a signal developed across the exciter field winding and the absolute value of the difference actuates a relay to give an alarm or shut down the machine.
The invention to be described hereinafter distinguishes over South, et al, in that the inventive contribution is not directed to a differential protection system of the type described by South, et al, and defined by the IEEE Standard Dictionary of Electrical Terms--IEEE Std.--1977 as published by The Institute of Electrical and Electronic Engineers, Inc. at page 178, which reads as follows: "Differential Protection. A method of apparatus protection in which an external fault is identified by comparing electrical conditions at all terminals of the apparatus." The invention to be described more fully hereinafter distinguishes over the South, et al contribution in that there provides a sensing of the voltage difference across the exciter field and a pairing of the voltage difference with generator load current the instant the voltage difference is sensed. A microprocessor is provided that has stored therein a range of preset values over a normal generator load current range such that when a voltage difference detected across the exciter field winding exceeds the stored value of the preset range of values, a relay is activated to interrupt the energization of the exciter field.